In the sanitary and household paper industry, these kinds of products are produced using a generally creped, absorbent, low basis-weight product featuring a design: viz., tissue paper. The structure's elongation capability, which is imparted by, for example, creping, is used to good advantage to emboss the sheet. This operation consists in permanently deforming the sheet between a non-deformable cylinder that is equipped with relief designs and a mating cylinder, for example, one that has a resilient coating. In this way projections are produced on one side that correspond to indentations on the other side.
With regard to hygiene products made of tissue paper, the trend in recent years has been to make them softer and gentler by manipulating their thickness and stiffness characteristics, especially by embossing. The embossing also makes it possible to visually improve the appearance of the product. The embossing operation is carried out on low-moisture-content paper, i.e., by transformation. This manipulation is thus done on a dry sheet unwound from a parent roll coming from the paper machine.
The most popular embossing patterns consist of a series of geometrically-based elementary projections having a small transverse section and a simple geometric shape. One example is described in U.S. Pat. No. 3,414,459, which pertains to a stratified sheet consisting of a plurality of elementary sheets, called plies, that are glued together. The plies are embossed with a distribution frequency and projection height that are adapted for the production of a water-absorbing product, for example, an all-purpose wipe. The number of elements ranges from 5 to 30 per cm2.
For his part, the Applicant has developed, especially for toilet paper, patterns with a larger number of elements, from 30 to 80 per cm2. This kind of embossing is usually described as “microembossing”. The relief elements that form the embossing necessarily have an element surface with a very low peak, less than 1 mm2. For these latter implementations, an appearance is obtained that is similar to that of a tissue product. An example thereof is disclosed in EP 426 548.
This kind of product has limited visual appeal, however. Moreover, when two plies that are embossed in this way are attached by gluing, the softness of the product that is obtained is less than optimal.
The solution offered in FR 2 728 152 makes it possible to improve the softness of a two-ply sheet with this kind of embossing by producing a combined pattern that includes a graphic pattern and a background pattern. The graphic pattern is made up of projections whose shape is linear, with a width of between 0.1 and 2 mm, and the background pattern consists of generally tapered small projections that are distributed at a ratio of at least 30 per cm2 (microembossing). The plies are connected along the linear pattern, thus limiting the extent of the glued-together surfaces and the degree of rigidity that is introduced.
In accordance with this embodiment, the vast majority of the embossed surface area is embossed according to the background pattern: 80% of the surface area in practice. This accounts for the vast majority of the functional characteristics associated with the embossing, i.e., thickness and absorbency.
U.S. Pat. No. 5,620,776 is also known from the prior art; it discloses a description of sheets of absorbent paper embossed according to a pattern that forms a grid of diamond-shaped figures whose interior includes a pattern that is composed of linear elements or is formed by aligned projections. The aim of this invention is to produce sheets with better swelling than non-embossed sheets, and sheets on which the patterns stand out better. However, the projection densities employed have nothing to do with microembossing.
EP 1 073 797 is also known from the prior art; it describes a sheet of absorbent paper that has a pattern composed solely of projections of equal size that are distributed according to nearly identical densities, such as to produce homogeneous marking and good definition of the pattern, while at the same time ensuring thickness and strength characteristics that are comparable to those of a sheet having uniform embossing.
The embossing pattern disclosed in this patent includes a network of cells whose interiors have little or no embossing.
In most cases, the goal is thus to strike a compromise between the density of the pattern-forming element(s), the surface area that they occupy, their shapes, their marking, etc.
The above-mentioned parameters can be manipulated depending on the effects that one wishes to achieve: softness, thickness, absorbency, aesthetics, strength.
The known kinds of microembossing have densities that are equal and/or essentially equal over the entire surface areas of the products.